Recent video compression standards (e.g., H.264) support multiple reference pictures for motion estimation and compensation. In general, a reference picture that correlates best to the current picture should be chosen. In interlaced video, each frame of video consists of two pictures: a top field and a bottom field. Whether a picture is a top field or a bottom field is referred to as its parity. Generally, the most appropriate options for the reference picture are the temporally previous field of the same parity, or the temporally previous field regardless of parity. In practical video encoders, it is not feasible to perform motion estimation using all possible reference pictures and choose the best result. Also, in order to take advantages of some implemental efficiencies, some video encoders can require that motion estimation be perform with respect to the same reference picture for every macroblock within a picture. Therefore, the reference picture must be selected before performing motion estimation for each picture.
It would be desirable to have a video encoder that automatically selects the most appropriate field picture to be used for motion estimation for field-encoded interlace video.